MongoClient not connected error while trying to use Mongoose?

Question

As part of my course I'm learning mongodb and now Mongoose. I've written the code exactly as done in the lesson but when trying to start it with node app.js I get the following error:

$ node app.js (node:25772) UnhandledPromiseRejectionWarning: MongoNotConnectedError: MongoClient must be connected to perform this operation at Object.getTopology (C:\Users\donal\OneDrive\Desktop\Udemy Web Development\FruitsProject\node_modules\mongoose\node_modules\mongodb\lib\utils.js:391:11) at Collection.insertOne (C:\Users\donal\OneDrive\Desktop\Udemy Web Development\FruitsProject\node_modules\mongoose\node_modules\mongodb\lib\collection.js:150:61) at NativeCollection.<computed> [as insertOne] (C:\Users\donal\OneDrive\Desktop\Udemy Web Development\FruitsProject\node_modules\mongoose\lib\drivers\node-mongodb-native\collection.js:200:33) at NativeCollection.Collection.doQueue (C:\Users\donal\OneDrive\Desktop\Udemy Web Development\FruitsProject\node_modules\mongoose\lib\collection.js:135:23) at C:\Users\donal\OneDrive\Desktop\Udemy Web Development\FruitsProject\node_modules\mongoose\lib\collection.js:82:24 at processTicksAndRejections (internal/process/task_queues.js:77:11) (Use node --trace-warnings ...to show where the warning was created) (node:25772) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag--unhandled-rejections=strict (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1) (node:25772) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

I have mongosh running in a separate tab in the terminal and I've tried several searches to look for similar problems. Any help very welcome!

Here's my app.js code in full:

mongoose.connect("mongodb://localhost:27017/fruitsDB");


const fruitSchema = new mongoose.Schema({
  name: String,
  rating: Number,
  review: String
});

const Fruit = mongoose.model("Fruit", fruitSchema);

const fruit = new Fruit({
  name: "Apple",
  rating: 7,
  review: "Pretty solid as a fruit"
});

fruit.save();
mongoose.connection.close();


const findDocuments = function(db, callback) {

  const collection = db.collection('fruits');

  collection.find({}).toArray(function(err, fruits) {
    assert.equal(err, null);
    console.log("Found the following records");
    console.log(fruits)
    callback(fruits);

  });
}

Answer 1

mongoose.connect is async, so you're trying to execute the code before a connection is established with the database.

const connectToMongo = async() => {
    await mongoose.connect("mongodb://localhost:27017/fruitsDB", {
        useNewUrlParser: true,
        useUnifiedTopology: true,
        useFindAndModify: false,
    });
    return mongoose;
};

You can now use async/await or .then() to wait till a connection is established

(async function () {
    await connecToMongo();

    const fruitSchema = new mongoose.Schema({
        name: String,
        rating: Number,
        review: String
    });

    const Fruit = mongoose.model("Fruit", fruitSchema);

    const fruit = new Fruit({
        name: "Apple",
        rating: 7,
        review: "Pretty solid as a fruit"
    });

    fruit.save();
    mongoose.connection.close();


    const findDocuments = function (db, callback) {

        const collection = db.collection('fruits');

        collection.find({}).toArray(function (err, fruits) {
            assert.equal(err, null);
            console.log("Found the following records");
            console.log(fruits)
            callback(fruits);

        });
    }
})

Answer 2

i ran into the same issue and the culprit is calling mongoose.connection.close(); in the body of your javascript. as @AnanthDev mentioned, mongoose is asynchronous and ends up closing the connection before it's able to perform the actions.

the following works because mongoose.connection.close() is in a callback function and does not run until after the actions is performed.

Fruit.insertMany([kiwi, orange, banana], function(err) {
    if (err) {
        console.log(err);
    } else {
        mongoose.connection.close();
        console.log("Successfully saved all the fruits to fruitsDB");
    }
})